Testicular biopsy specimens obtained from three patients with Klinefelter's syndrome were studied by light and electron microscopy. Of note in the ultrastructural findings were the absence of annulate lamellae in the Sertoli cells and the abnormal mitochondria, the presence of microcrystalline formations, and the absence of the crystals of Reinke in Leydig's cells. It is suggested that the impaired hormonal function of the testes in Klinefelter's syndrome may be related to the absence of annulate lamella in the Sertoli cells and the presence of microcrystal formations in Leydig's cells.